[Gdal-dev] Breaking long lines/boundaries with OGR function?

[Ari Jolma]

On Mon, 26 Jun 2006 10:16:06 +0200 Markus Neteler <neteler at itc.it> wrote:

> Ari,
> 
> of course the pure import is fast, but the reconstruction
> of topology takes time. Our idea is to deliver the data
> in a way more suitable to fast topology reconstruction.
> It's all about topology - I guess that your renderer
> doesn't built it.

No, I suspected something like this. Then the bottleneck is in the usage of the
topology code, which is GEOS in the case of OGR, I believe. Topological
operations on complicated geometries are more or less inherently slow.

In v.in.ogr you have more or direct access to OGR data and you can easily split
the data into smaller pieces for your own use. Since this splitting is an
optimization which depends on the need and there is no "logical" way for doing
it generally, I don't think there is a suitable existing function in OGR for
doing it. At least I can't think of anything now. Of course I maybe wrong and
Frank points out something that I can't think now ;)

Ari

> 
> Hope this clarifies the idea (I'm no topology expert...),
> 
>  Markus
> 
> 
> On Mon, Jun 26, 2006 at 10:53:24AM +0300, Ari Jolma wrote:
> > Markus,
> > 
> > I'm not sure why the import should be effected by the spatial extents
> etc. In my
> > code I "import" data from OGR each time I render it and there's no
> big overhead
> > because of that. The code is also extremely simple. What would you
> consider
> > "complicated"? I've for example looked at ghssh coastline data and
> they render
> > reasonably fast.
> > 
> > Of course I don't know much about the way GRASS stored vector data or
> about the
> > import tool :-)
> > 
> > Ari
> > 
> > On Mon, 26 Jun 2006 09:11:35 +0200 Markus Neteler <neteler at itc.it> 
> wrote:
> > 
> > > Hi,
> > > 
> > > the following issue came up in GRASS:
> > > v.in.ogr, the OGR based import tool, may take a long time if
> > > the data contains long complicated polylines.
> > > 
> > > >From TODO
> > > http://freegis.org/cgi-bin/viewcvs.cgi/grass6/doc/vector/TODO
> > > > ============================
> > > > v.in.ogr
> > > > --------
> > > > It would be useful to split long boundaries to smaller
> > > > pieces. Otherwise cleaning process can become very slow because
> > > > bounding box of long boundaries can overlap large part of the map
> > > (for
> > > > example outline around all areas) and cleaning process is
> checking
> > > > intersection with all boundaries falling in the bounding box.
> > > > ============================
> > > 
> > > Since I don't know OGR well enough, is there any function 
> > > we could make use of to achieve splitting of long lines before
> > > handing data over to (topological) GRASS?
> > > 
> > > I found 
> > > http://ogr.maptools.org/drv_dgn.html
> > > "Polygons and line strings with too many vertices will be split
> into
> > >  a group of elmements prefixed with a Complex Shape Header or
> Complex
> > >  Chain Header element as appropriate."
> > > 
> > > Something seems to be there (or nearby).
> > > 
> > > thanks for a hint
> > > 
> > >  Markus
> > > _______________________________________________
> > > Gdal-dev mailing list
> > > Gdal-dev at lists.maptools.org
> > > http://lists.maptools.org/mailman/listinfo/gdal-dev
> > > 
> > 
> > -- 
> > Prof. Ari Jolma
> > Kartografia ja Geoinformatiikka / Cartography and Geoinformatics
> > Teknillinen Korkeakoulu / Helsinki University of Technology
> > POBox 1200, 02015 TKK, Finland
> > Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma
> 
> -- 
> Markus Neteler  <neteler itc it>  http://mpa.itc.it/markus/
> ITC-irst -  Centro per la Ricerca Scientifica e Tecnologica
> MPBA - Predictive Models for Biol. & Environ. Data Analysis
> Via Sommarive, 18        -       38050 Povo (Trento), Italy
> 

-- 
Prof. Ari Jolma
Kartografia ja Geoinformatiikka / Cartography and Geoinformatics
Teknillinen Korkeakoulu / Helsinki University of Technology
POBox 1200, 02015 TKK, Finland
Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma